Automatic gain control (AGC) is an essential element in modern communication receivers enabling the estimation of signal strength and the automatic adjustment of the amplification gain in order to maintain a relatively constant signal level. In typical conventional systems, such as in burst-mode wireless communication systems, for example, the Bluetooth system, the received signal needs to be level-adjusted prior to further processing. Furthermore, it is desirable to have a signal with a relatively constant strength (amplitude) to enable the full range of the processing circuits to be used. Moreover, the correct operation of analogue-to-digital converters can only be achieved if the excursion of the input signal is within the converter-specific dynamic range, thus avoiding saturation.
Automatic gain control circuits monitor the strength of the signal applied to the subsequent processing circuits, for example, an analogue-to-digital converter. A feedback signal is generated to readjust the gain of the amplifiers in the receiver stages, in a self-adaptive manner.
Conventionally, the feedback signal is typically generated by analogue circuitry. In systems where the signal is applied to an analogue-to-digital converter, if the level of the signal being applied to the analogue-to-digital converter exceeds the dynamic range of the analogue-to-digital converter, the control loop will have difficulty in reaching a steady state. This problem is particularly evident in applications where the received signal consists of packets of data with a short header code (preamble) during which time the automatic gain control circuitry needs to settle.
In the Bluetooth system, the short (4-bit) preamble allows only a very short settling time (4 microseconds) for the automatic gain control circuitry to settle prior to processing of the data packet. Traditional automatic gain control circuits in most modern receivers have difficulty achieving precise gain adjustment of the received signal to a desired level within such a short response time.
Thus, there is a need for an improved automatic gain control system and method having a faster response time than that available in conventional systems.